Incidents of large-amplitude vibrations of stay cables have been reported worldwide when certain combinations of light rain and moderate winds (for instance, ten to fifteen m/s) exist. Stay-cable vibrations having amplitudes on the order of one to two meters have been experienced. This aerodynamic phenomenon is known as "rain-wind induced vibration" (at times referred to as "wind-rain induced vibration" or just "wind-rain vibration") and it is a widespread problem. This problem is believed to have been first identified during the late 1980's to early 1990's time period.
The amplitude of vibration is the maximum degree of vibration (oscillating movement and its repetitions) that will be suffered by a cable. The larger the vibration amplitude and its repetitions, the greater is the adverse effect on fatigue endurance of cables, particularly at their end anchorages.
The cause of these unexpected large-amplitude vibrations (which are believed to be a form of aerodynamic instability) is believed to be the formation of water rivulets on the cables. Such type of large-amplitude vibrations has not been seen in the absence of either the light rain condition or the moderate wind condition. The stay cables are the primary load-carrying members of cable-stayed bridges, and thus they are at least one of the most important and crucial elements of the entire bridge structure. When the stay cables vibrate, the bridge as a whole will generally vibrate. Therefore, rain-wind induced vibration of stay cables can be highly detrimental to the long-term health of stay cables and cable-stayed bridges. The rain-wind induced vibrations that have been experienced were not among the generally anticipated types of cable vibrations such as vortex, galloping and wake galloping. Vibration-induced fatigue stemming from rain-wind induced vibration therefore was also not previously anticipated. Such large-amplitude vibrations can significantly affect the strength and the useful life of the cables, and thus in turn affect the strength and the useful life of the bridge.
Because of (a) the large amplitude of the stay-cable vibration that is induced by rain-wind conditions, (b) the inducement of rain-wind vibration by conditions that are neither rare nor extraordinary, (c) the structural importance of cables to cable-stayed bridges, and (d) the typical vast span of cable-stayed bridges, rain-wind induced vibration of the cables is a source of great concern for the bridge engineering community, and a source of deep public anxiety.
In general, a number of different types of cable vibration control measures have been utilized in cable-stayed bridges. These vibration-control measures include neoprene washers (also known as neoprene rings), cross cables (also known as cross ties or cable ties), hydraulic dampers (also known as external mechanical viscous dampers), and modified polyethylene sheathing. Neoprene washers are a commonly used control measure which, in more detail, are placed in the annular space between the outside diameter of the cable and steel guide pipes near the cable's low and high anchorages, the guide pipes being attached respectively to the bridge deck and to the pylon. The level of cable damping achieved by neoprene washers is highly dependent on the tightness of fit, the level of precompression, and any confinement for the neoprene, and thus their damping contributions are highly variable and not easily predictable. Cross cables transversely connect different stay cables together. Besides introducing cross-cable and tie-connection design and fatigue issues, (1) cross cables negatively impact bridge aesthetics, (2) there has a yet to be established any cross-cable damping contribution of significance, and (3) failures of the cross cables themselves have been experienced. Mechanical viscous dampers rely on reaction of the damper force against the bridge deck, and thus they are generally mounted at the cable's low end, diminishing the level of damping attainable. Polyethylene sheathing, modified to include surface irregularities (protrusions, dimples, spiral strakes) to disrupt the water rivulets, require special fabrication and the effects of such modifications on drag coefficients need particular attention. All of these known measures suffer from one or more drawbacks, including (a) a maximum attainable damping below that desired (does not meet desired performance requirements), (b) a maintenance burden greater than desired, (c) high fabrication costs, (d) high installation costs, particularly in retrofit installations, (e) an abatement of, or detriment to, bridge aesthetic issues, and combinations of these drawbacks.